The present invention relates to a method for the production of a silicon single crystal used as a semiconductor by the chemical vapor deposition technique and a method for the fractional determination of ultratrace elements (such as phosphorus, arsenic, boron and aluminum) present in chlorosilanes as starting materials for the production of the silicon single crystal and the resulting silicon single crystal.
To produce a silicon single crystal for use as a semiconductor by the chemical vapor deposition technique, a starting material such as chlorosilanes or monosilane is supplied to a solid phase comprising a polycrystalline silicon seed held in a growth chamber while heating the starting material together with a reducing component such as hydrogen gas to grow a polycrystalline silicon crystal on the polycrystalline silicon seed, then the grown polycrystalline silicon crystal is withdrawn from the growth chamber and the polycrystalline silicon is converted into a silicon single crystal according to the floating zone melting method (FZ method). Silicon semiconductor wafers are produced from the silicon single crystal thus produced.
The silicon single crystal thus produced comprises, as impurities, ultratrace elements such as phosphorus, arsenic, boron and aluminum which have been included in the chlorosilanes used as the starting material. Electrical properties of the resulting semiconductor are greatly affected by these impurities. For this reason, the accurate fractional determination of these ultratrace elements present in silicon single crystals are closely related to the quantitative determination of ultratrace elements in starting materials and is quite important for evaluating the electrical properties of the resulting semiconductor.
Method F574-83 of the American Society for Testing and Materials (ASTM) is a method for determining the amounts of trace elements present in silicon crystals for semiconductors. This method comprises producing a silicon polycrystal according to the chemical vapor deposition technique, converting the resulting polycrystal into a silicon single crystal according to the floating zone melting method, determining the specific resistance of the single crystal, then repeating several times the floating zone melting procedures under vacuum to remove donors (phosphorus and arsenic) and again determining the specific resistance of the single crystal to evaluate the concentration of the sum of the remaining acceptors (boron and aluminum). The concentration of the sum of phosphorus and arsenic can be calculated on the basis of the total concentration of the acceptors and the difference between the specific resistances as determined above.
However, the silicon single crystal conventionally obtained by the combination of the chemical vapor deposition and floating zone melting methods would possibly be contaminated by the floating zone melting apparatus per se and accordingly it is difficult to obtain silicon single crystals having sufficient high purity. Moreover, the floating zone reel ting method requires complicated operations, skill in the operator while taking a long time period to obtain a silicon single crystal.
Further, the foregoing conventional method for determining the amounts of trace elements suffers from the following problems and is improper for quantitative determination of ultratrace elements in which highly accurate measurement is required. First of all, the concentrations of ultratrace elements present in the silicon polycrystal grown according to the chemical vapor deposition technique often differs from lot to lot. If silicon polycrystals are continuously treated in the floating zone melting apparatus to give single crystals, the concentration of the trace elements present in the silicon polycrystal previously treated to convert a single crystal, would greatly affect the silicon polycrystal subsequently treated by the apparatus. Moreover, phosphorus, arsenic and boron cause segregation in the direction along which the floating zone melting method is performed. Accordingly, the practical concentration of these trace elements should be determined while taking the influence of the segregation into consideration. In addition, the specific resistance becomes great when the amount of trace elements is extremely small and in the worst case, for instance, if the specific resistance is greater than about 5000.OMEGA..multidot. cm, Joulean heat is generated which leads to an error in the measurement of specific resistance. Finally, the foregoing method does not give the concentration of each individual trace element, but gives the concentration of the sum of phosphorus and arsenic and that of the sum of boron and aluminum.